The present invention relates to a vessel for laying underwater rigid tubular pipes and flexible pipes intended, in particular, for offshore oil production.
The person skilled in the art makes a distinction between flexible pipes and rigid pipes. Specifications API 17B and 17J by the American Petroleum Institute can be referred to for a definition of flexible pipes, and specification API 17A can be referred to for a definition of rigid pipes. It is simply recalled that flexible pipes have a minimum bend radius without damage (often termed MBR) which is relatively small (for example a few meters) while as opposed to rigid pipes have a minimum bend radius without plastic deformation which is relatively large (for example several tens of meters). Furthermore, for the purpose of the description below, the general term flexible pipes will be used to encompass not only flexible tubular pipes in their strictest sense, but also flexible risers, umbilicals and flexible cables that a laying vessel may have to lay.
Laying systems for rigid pipes are described in API 17A and for flexible pipes are described in API 17B.
In the case of rigid pipes, a distinction is made between two types of laying, depending on whether the sections of pipe are welded at sea (stove piping) or welded on land (reeled pipe).
The first type of laying itself has three main subdivisions, S-lay, J-lay and G-lay.
According to the S-lay technique, the sections of pipe are assembled horizontally on the deck of the vessel and they are made to follow an S-shaped path with a very large radius of curvature so as to prevent any plastic deformation. The pipe leaves the vessel at a very oblique angle, with the aid of an inclined trough known as a stinger. This technique is suited only to shallow and moderate laying depths because otherwise the weight of the string of pipe already laid would run the risk of bending and plastically deforming the pipe at the point where it leaves the stinger. This is why other techniques have had to be developed for greater depths of water.
According to the J-lay technique, the sections of pipe are assembled on a vertical or very slightly inclined davit on the laying vessel, the sections thus being welded together in the direction in which the pipe is laid, which avoids any plastic deformation at this stage, the assembled pipe then being immersed to a great depth, still without plastic deformation. Examples are illustrated in document U.S. Pat. No. 5,464,307 or in French application 98/00148 in the name of the assignee company.
According to the G-lay technique, illustrated, for example, in document GB 2 287 518 A, sections of pipe are assembled horizontally on the deck of the vessel as in the S-lay technique; the pipe formed is sent towards the bow of the vessel then is returned aft by a large wheel which introduces plastic deformation into the pipe, after which this pipe passes through straighteners and a series of multi-track caterpillar tensioners and is then dropped into the water with a certain inclination which can be chosen according to the depth of water so as to avoid plastically deforming the pipe in the submerged part. According to one alternative form disclosed in GB 2 296 956 A, the pipe, previously assembled from sections on the deck, is sent over an arched chute placed at the top of an inclinable ramp equipped with tensioners or clamping jaws; the two inflexions that the pipe undergoes lead to plastic deformation of the pipe and make it necessary to employ straighteners. The rate of laying using the G-lay technique is relatively slow.
The second type of laying for rigid pipes, using sections welded together on land, comprises the so-called reeled pipe system illustrated, for example, in GB 2 025 566 A, where the pipe, which has been welded together as a long length (in excess of 1 km for example) on the dockside and wound with plastic deformation onto a vertical reel (which may be as much as 15 to 20 m in diameter) located on the laying vessel, is unwound from the reel, again with plastic deformation, and passes through a straightener-reeler before being taken up by the tracked-caterpillar conveyors of an inclinable laying ramp provided in a zone where the pipe enters the water at the stern of the vessel and the inclination of which is chosen according to the laying depth. This technique does, however, have limitations in terms of the depth achieved. At the present time, the sea beds being exploited are often at depths in excess of 2000 m, which demands that a considerable length of rigid pipe be carried on board. This is something of which existing vessels are incapable.
In the case of flexible pipes, a distinction is also made between two types of laying: the horizontal laying system (HLS), the application of which is soon limited to shallow laying depths; and the vertical laying system (VLS). FR 2 660 402 A in the name of the assignee company discloses such VLS-type laying from the central well (moon pool) of a laying vessel. The pipe is unwound from a storage basket and sent over a chute which guides it in a circular arc (termed, in common parlance, a xe2x80x9ccamembertxe2x80x9d) located at the top of a vertical derrick of modest height surmounting the moon pool and supporting two multi-track caterpillar tensioners which clamp and move the pipe along continuously. This system is well suited to flexible pipes (even though the depths reached are currently about 1700 m), that is to say the pipes which can adopt a fairly pronounced curvature both in the storage basket and over the return chute and on the intermediate portions of the caternary-curve-shaped part. By contrast, this system is not suitable for rigid pipes which cannot withstand such curvatures or in which such curvatures lead to permanent deformation.
As can thus be seen, the current laying ships are not suited to the laying of rigid and flexible pipes. A ship for laying reeled rigid pipe can be used for laying flexible pipe but this means of laying is not optimized for the laying of flexible pipe (the radius of curvature given to the flexible pipe during laying is far greater than the minimum bend radius of the flexible pipe). It is also possible to use the system described in French patent application 98/00148, but the J-lay davit has to be modified for laying the flexible pipe. Admittedly, there are some laying ships (for example SEAWAY FALCON and MAXITA) which can lay both rigid and flexible pipes, but the laying means on this type of ship are independent (there is one davit for laying rigid pipe and another for laying flexible pipe).
The object of the invention is to propose a vessel for laying reeled rigid pipe which allows laying at very great depths (particularly 2500 m and over) without running into the limitations hitherto encountered. Another object is to propose a vessel capable of combining the reeled pipe system with the system (VLS) used for laying flexible pipe by sharing the same laying means, and even that of the rigid-pipe J-lay system.
The objective of the invention is achieved using a vessel for laying rigid tubular pipes, of the type comprising means of storing tubular pipe wound with plastic deformation on vertical reels, an inclinable laying davit arranged approximately over a zone at which the pipe enters the water, an arched chute for deflecting the pipe at the top of the davit, and pipe-straightening means and tensioners arranged on the davit, characterized in that the storage means are in the form of two adjacent vertical wheels of very large dimensions facing approximately towards the davit, the mid-planes of the wheels intersecting in the vertical longitudinal mid-plane of the vessel.
The invention solves the problem of increasing the capacity of the vessel for storing wound rigid pipe in an original way. Rather than increasing the dimensions of a single storage wheel in proportions which would make it difficult to control (for example, a wheel weighing 500 tons presents problems of rotational inertia and demands substantial reeling means), the invention proposes to arrange two adjacent storage wheels in a particular orientation, which makes it possible not only to control the problems of rotational inertia, but also to dispense with reeling means for unwinding the wound rigid pipe especially if, as is preferred, the deflecting chute can itself be directed towards one or other of the wheels. For this purpose, the top of the davit is mounted in such a way as to allow sufficient pivoting to each side of the longitudinal mid-plane of the vessel, for example by about 15xc2x0, and to do so when the davit is vertical or inclined by an angle which may be as much as 60xc2x0 to the horizontal. The pivoting top of the davit contains the straightening means (preferably at three points) so that the straightening of the unwound rigid pipe always occurs in a correct orientation without introducing additional false twist due to the angular position of the vertical storage wheels (that is to say plastic deformation in a plane different from the normal plane of curvature of the wound pipe).
The mid-planes of the wheels ideally intersect at the point where the rigid pipe enters the straightening means of the davit. As this davit is of variable inclination (between 60xc2x0 and 90xc2x0), it may be envisaged for the angular position of the wheels to be adjustable according to the laying angle, using appropriate means for supporting and orienting the hubs of the wheels. In a simplified version, the wheels are given a fixed orientation corresponding to the ideal position for an average laying angle (75xc2x0) which, in one exemplary configuration, amounts to making the mid-planes of the wheels intersect forward of the zone at which the pipe enters the water, approximately in the region of the top of the deflecting chute (when the davit is in a vertical position). In practice, the wheels then make an angle of about 5xc2x0 with respect to the mid-plane of the vessel.
In addition to the advantages already mentioned, the fact of having two storage wheels makes it possible to halve the time taken, on the quayside, to load the wheels and makes it possible to store different pipes on each of the wheels, for example pipes of different diameters.
According to the invention, the zone at which the pipe enters the water is a moon pool located roughly at the center of the vessel. At the moon pool there is an independent table comprising a clamp. This clamp is able to hold the string of pipe while accessories are affixed to the pipes or while two sections of pipe are being connected. This clamp can also be used as a static clamp for J-laying. For this purpose, a special-purpose ramp for J-laying is attached to the davit. The aft deck is used for preparing sections of rigid pipe. J-laying is carried out using a mobile carriage mounted on the special-purpose ramp and operated by the abandonment and recovery winch, and using the static clamp.
Naturally, the vessel also has a well for the passage of an underwater laying robot.
To allow the vessel the desired multi-functionality, flexible-pipe storage baskets are provided on the vessel, and the davit also has, at its upper end, a chute for flexible pipe. The davit can therefore be used for laying flexible pipe using the VLS technique. Furthermore, enough space is provided at the laying table at the base of the davit for affixing various accessories (vertebras, buoys, etc.). The stern of the vessel is also large enough to accommodate a great deal of tackle and equipment needed for preparing the pipes.